The present invention relates to a method for forming a thin film on the surface of a substrate with a gas cluster ion beam. More particularly, the present invention relates to a method for forming a thin film by a gas cluster ion beam which is a group of reactive substances gaseous at the room temperature and under atmospheric pressure. The method is useful for the manufacture of a semiconductor or other electronic devices and for surface reforming of a functional material.
A method for forming a thin film by irradiating monatomic or monomolecular ions onto a substrate surface has conventionally been used in practice. This method utilizes a high input energy of several keV because low-energy ion irradiation cannot give an adequate beam because of the space charge effect between ions.
In this conventional method, however, the use of ions having a large input energy makes it difficult to avoid damage to the substrate surface, and thus deterioration of semiconductor devices has been a major problem.
As a method for forming a thin film by causing reaction between a reactive substance, which is gaseous at the room temperature, and a semiconductor substrate, a thermal reaction method has been developed for forming an oxide or nitride film by heating the substrate to a high temperature in an atmosphere of the reactive substance. The thin film formed by this method has excellent interface and insulation properties, and the method has been industrialized for forming an insulating film or a capacitor insulating film for silicon semiconductor devices.
This method requires a low temperature to reduce the diffusion of impurities, which is desirable for integrated circuit devices, and the chemical vapor deposition (CVD) method is adopted for this purpose.
In this case, however, it is necessary to heat the substrate surface to a temperature of at least 400xc2x0 C., but this results in defects such as a low density of the resultant film as compared with that produced by the thermal reaction method and the presence of many unsaturated bonds on the interface between the substrate and the thin film.
The plasma CVD method is known as a method for forming a thin film at a low temperature. This method however involves such defects in that it introduces a large amount of mixed impurities in the substrate, causes damage to the substrate surface by the ions, and results in difficulties controlling the film thickness of an ultra-thin film. Furthermore, it is not applicable to a transistor gate insulating film or a capacitor insulating film requiring high quality.
In the conventional techniques, as described above, the quality of the resultant thin film deteriorates as the process temperature becomes lower, and it is difficult to obtain an ultra-thin film useful for a hyperfine semiconductor circuit device such as ULSI.
There has consequently been a strong demand for a new method which permits formation of a high-quality thin film at a lower temperature, particularly at room temperature, without the need to heat the substrate and without damaging the substrate surface, as a fundamental technology for use in advanced electronics such as ULSI.
The present invention was developed in view of the circumstances as described above, and has an object to provide a method for forming a high-quality ultra-thin film having a smooth interface with the substrate free from a damage at the room temperature, which solves the drawbacks in the conventional methods for forming a thin film.
The present invention provides a method for forming a thin film on the surface of substrate with a gas cluster ion beam, which comprises the step of irradiating the surface of a substrate with ions of a gas cluster (which is a massive group of atoms or molecules of a reactive substance in gaseous form at room temperature and under atmospheric pressure) to cause a reaction with a substance of the substrate surface, thereby forming a thin film on the substrate surface.
More specifically, the invention comprises the steps of using a reactive substance selected from the group consisting of oxides, nitrides, carbides, mixtures thereof, and mixtures thereof with an inert gas and causing a reaction between a gas cluster ion beam of this substance and a substance of the substrate surface, thereby forming a thin film on the substrate surface.
The invention provides also a method for forming a thin film based on a gas cluster ion beam, which comprises the steps of irradiating the surface of the substrate with a gas cluster ion beam to form a thin film and at the same time to planarize the surface.